1. Field of the Invention
The present invention relates to an adaptive equalizer and an adaptive equalization method, and more particularly, to an adaptive equalizer for equalizing an input signal to a signal having the level required by a decoder, and an adaptive equalization method performed by the equalizer. The present application is based on Korean Application No. 2001-78275 which is incorporated herein by reference.
2. Description of the Related Art
To equalize an input signal transmitted through a channel having continuously changing characteristics into a desired type of signal, an equalizer having an FIR filter function for equalizing the input signal is generally used.
FIG. 1 is a block diagram showing a conventional adaptive equalizer having an FIR filter. Specifically, an adaptive equalizer for a high-speed optical disk system introduced in EP1014363 titled “Reproducing Apparatus” is shown in FIG. 1. As shown in FIG. 1, the conventional adaptive equalizer has an FIR filter 10, a zero detector 14, a tap delay circuit 16, a temporary decision block 12, and a coefficient calculator 18.
The FIR filter 10 generates an equalized output signal by equalizing the input signal. In an optical disk system, the input signal is an RF signal read by a pick-up from a disk such as a DVD or a CD. The output signal output from the FIR filter 10 is provided to a viterbi decoder (not shown).
The zero detector 14 detects zero value of the equalized output signal output by the FIR filter 10. The zero detector 14 outputs ‘1’ as a detection level value when the value of the equalized output signal has a value close to ‘0’. Otherwise, the zero detector 14 outputs ‘0’ as the detection level value.
The tap delay circuit 16 creates a series of zero detection values by delaying a zero detection signal detected by the zero detector 14. In other words, the tap delay circuit 16 creates the zero detection values (for example, 01000100010) by delaying the value of ‘0’ or ‘1’ created by the zero detector 14 by using a delaying element.
The zero detection values from the tap delay circuit 16 are input to the temporary decision block 12, and also the equalized output signal from the FIR filter 10 is input to the temporary decision block 12. The temporary decision block 12 decides an error of the equalized output signal by using the zero detection values and the equalized output signal. In other words, the temporary decision block 12 detects a level of the output signal of the FIR filter 10 using the zero detection values. After that, the temporary decision block 12 calculates the error by calculating a difference between the detection level and a real level of the output signal of the FIR filter 10.
The error from the temporary decision block 12 is input to the coefficient calculator 18, and also a signal achieved by delaying the input signal for a predetermined time is input from the FIR filter 10 to the coefficient calculator 18. The coefficient calculator 18 calculates new coefficients to be input into the FIR filter 10 by using the error, the delayed input signal and the present coefficients of the FIR filter 10. At this time, the coefficient calculator 18 applies an LMS (Least Mean Square) algorithm to the error and the delayed input signal, calculates the coefficients required by the FIR filter 10, and provides the calculated coefficients to the FIR filter 10.
The FIR filter 10 is modified to have various features in accordance with coefficients created by the coefficient calculator 18. Accordingly, the FIR filter 10 can equalize the input signal to output a signal having a desired output level.
As described so far, the conventional adaptive equalizer uses the zero detection value and the equalized output signal to allow the temporary decision block 12 to calculate the error. Therefore, the credibility of the error is determined by the accuracy of the zero detection value.
However, in the conventional adaptive equalizer, since the zero detector 14 detects as zero a change in sign considering only the sign of the equalized output signal, there might be a problem that a frequent malfunction can occur when the zero detector 14 detects zero in the case that there are a lot of changes in the signal input into the zero detector 14.
For example, when the state of the channel through which the input signal is transmitted is not satisfactory, as for example, when there is much noise or a phase error in the input signal, there might be undesired changes to the input signal. Especially, when the level of the input signal frequently changes in the area around zero due to the noise included in the input signal, the zero detector 14 outputs the output signal of ‘1’. Therefore, the temporary decision block 12 cannot calculate the error accurately, and the coefficient calculator 18 is mis-operated. Consequently, the entire capability of the equalizer is deteriorated.
Furthermore, the deterioration of the input signal causes divergence of the coefficients, thus a serious error can occur in the operation of the system. Especially, when the conventional adaptive equalizer is used in a system such as an optical disk system having frequent abrupt deterioration of the signal, a serious problem can be generated in the stability of the system. The distortion of the signal in the optical disk system can be caused by several factors, mainly by fingerprints or foreign substances clinging to the surface of the optical disk due to carelessness of a user. Moreover, distortion of the input signal can be generated in the case that the RF signal is not input at all, such as when an area to be read is changed from a certain area of the disk to another area.
When an abnormal signal is input into the adaptive equalizer, the coefficients of the equalizer may diverge. Therefore, although a normal input signal is received afterwards, it might take a long time for the input signal to returned to normal coefficients, or the input signal cannot be returned to normal coefficients.